The major objective and principal scientific challenge of this proposal is the implementation of new methodology for the stereocontrolled synthesis of building blocks for a number of biologically important natural products, based on novel organomolybdenum chemistry. Some key targets chosen so as to highlight the considerable potential of this new approach to organic synthesis are a C30 diol obtained from messel shale kerogen, the side chain of vitamin E and the right hand segment of an important macrolide antibiotic, magnamycin B. When properly developed, the approaches described may be applied to the synthesis of unnatural analogs of these substances leading to compounds having beneficially modified biological activity. The method to be developed is based on the ability of the dicarbonylcyclopentadienylmolybdenum cation, when attached to a diene, to activate the diene to nucleophilic attack. Successive carbon nucleophile addition, reconversion of Pi-allylmolybdenum products to diene complexes followed by second nucleophile addition generates a stereodefined disubstituted - - allylmolybdenum complex which can be converted to organic products using novel methods developed in our laboratory. This unique approach to stereocontrol is to be used in six-, seven- and eight-membered rings to generate the proposed target molecules. Of particular significance is the influence which we can expect the dicarbonylcyclopentadienylmolybdenum moiety to have on the conformations of awkward ring sizes, thereby allowing controlled manipulations which are not possible without the aid of the metal.